Generally speaking, because automobiles emit volatile fuel vapors into the atmosphere, regulations were established requiring fuel vapor management systems to be included in all automobiles so as to prevent or otherwise reduce these emissions. Typical fuel vapor management systems include a carbon filled canister to absorb unburned fuel vapors, and a centrally managed vent and valve control arrangement for routing flow of the fuel vapors to either the carbon filled canister or to the engine intake for proper combustion therein.
In addition, regulations have also been established which require such fuel vapor management systems to include an on-board diagnostic capability for detecting the existence of leaks within the system. An example of one such diagnostic system is commonly owned U.S. Pat. No. 5,261,379 to Lipinski et al. In general, during testing operation, these systems apply a partial vacuum to the fuel tank of the vehicle until a predetermined pressure level is reached. Once the predetermined pressure level is reached, the tank is sealed, and the system measures the amount of vacuum "bleed off" or change in pressure over a predetermined period of time.
However, performance of typical diagnostic systems is a direct function of such factors as tank flex, fuel temperature, heat added by circulated fuel and ambient or underbody air temperature, fuel RVP (Reid Vapor Pressure), and age of the fuel. In addition, because of new regulations requiring diagnostic systems to be able to validly detect a leak equivalent to a 0.020 inch (0.051 cm) diameter hole, such factors have become even more problematic to producing reliable test results.
Detection of fuel vapor pressure is employed in some diagnostic and fuel vapor management systems as part of the overall process for detecting the presence of leaks. One way of attempting to quantify fuel vapor pressure is to measure fuel temperature, and then calculate fuel vapor by correlating the temperature measurement with a predefined table showing which fuel vapor pressure should be present for a particular fuel at a given temperature. The problem with such an arrangement is that there is no accommodation for the effects of fuel RVP and age, which materially alter the relationship between fuel vapor pressure and fuel temperature.